There is a risk in aircraft fuel systems of a build up of static electricity on one component in comparison to another component to which it is connected. In the prior art, a so-called bonding lead is connected to each side of the joint in order to electrically connect the components and prevent static build up. An example of such a bonding lead is described in the prior art section of U.S. application Ser. No. 2008/0078880.
U.S. application Ser. No. 2010/0226063 discloses bonding leads which are formed from a material with a relatively low conductivity. This results in a relatively low flow of current which reduces the risk of sparking in the event of a lightning strike. However this requires the electrical conductivity of the material forming the bonding lead to be changed depending on the overall length of the bonding lead. This makes it difficult to provide bonding leads in different lengths which nevertheless lie within the desired resistance range.
A further bonding lead arrangement is described in U.S. application Ser. No. 2010/0226062. In this case the bonding lead comprises a conductive core and a sheath which covers a substantial length of the core and has a lower electrical conductivity than the core. The path of least electrical resistance between the first component and the core of the bonding lead comprises the sheath.
SUMMARY OF THE INVENTION
A first aspect of the invention provides a joint according to claim 1. A second aspect of the invention provides a method according to claim 12.
Typically the electrical resistance of the bonding lead is much lower than that of each washer, typically by a factor of more than 100, so the total resistance of the assembly remains substantially the same regardless of the length of the bonding lead.
The shaft of the fastener may engage a wall of the hole in the bonding lead. However this may result in a low resistance electrical path via the shaft which makes the overall resistance of the assembly too low. Therefore more preferably each washer has a sleeve which surrounds the shaft and is received in the hole of the component and an annular flange which extends outwardly from the sleeve, wherein the flange of the washer engages the component and the inner face of the bonding lead and provides said path of least electrical resistance. This arrangement ensures that there is no engagement between the bonding lead and the shaft of the fastener, making it easier to control the electrical conductivity of the connection assembly since this will depend only on the material and geometry of the washer flange.
Optionally each connection assembly may further comprise a second washer with a sleeve which surrounds the shaft and is received in the hole of the component and an annular flange which extends outwardly from the sleeve, wherein the flange of the second washer engages the outer face of the bonding lead. The sleeve of the second washer may engage the sleeve of the first washer, or there may be a gap between them.
Preferably each connection assembly further comprises a nut which is carried by the shaft and can be tightened to compress the washer between the component and the inner face of the bonding lead. The nut may engage the second washer, and the second washer may be resiliently compressed by the nut.
The bonding lead may dissipate electrostatic charge across the joint and/or conduct lightning current across the joint.
One or both of the components in the joint may be a fuel pipe, a hydraulic pipe, an air pipe, a wall of a fuel tank, an access panel in a wall of a fuel tank, or any other part of an aircraft which is in contact with fuel when in use (in liquid or vapour form) or has the potential of doing so.
The electrical assembly provides an electrical pathway between the first and second components with a relatively high electrical resistance—between 100 kΩ and 10 MΩ, preferably between 200 kΩ and 1 MΩ, and most preferably between 200 kΩ and 500 kΩ.